Data on everyday computers are comprised of bits, which are a binary sequence of zeroes and ones. These bits can be stored magnetically on hard drives or electrically on flash drives. In the last few decades quantum information processing using quantum bits or “qubits” has emerged as a completely new form of computation. Unlike a bit, a single qubit is a quantum mechanical object and can be in a combination or superposition of zero and one states. Qubits can be manipulated and processed to perform computational tasks.
Two common ways to represent a qubit include: using the quantized angular momentum, or spin, of a charged particle, e.g. spin up=0 and spin down=1, or by using photons, e.g. no photon present=0, one photon present=1.
It has been suggested that semiconductor quantum dots (QDs) are excellent candidates for stationary qubits. The large oscillator strengths of their optical transitions allow for quick initialization, optical manipulation, and read-out of the spin state. In addition, the solid-state nature of these emitters makes it possible to structure their environment on the nanometre length scale, thus allowing engineering of their photonic local density of optical states (LDOS) through fabrication of photonic-crystal structures. For guiding light along certain directions, such photonic crystals typically utilise a geometry where a nanostructure is arranged mirror symmetric (or up-down symmetric) about a guiding region. However, due to the symmetry of such photonic-crystal waveguides, these waveguides have electric fields that are even or odd functions with respect to the guiding region and are dominated by electric fields that are linearly polarized. Therefore, the different spin states of quantum dots or any integrated quantum emitter with circularly polarized transition dipole moments cannot efficiently couple to such waveguides.
Adam Mock et al.: “Space group theory and Fourier space analysis of two-dimensional photonic crystal waveguides”, Physical Review B, Vol. 81, no. 14 discloses type B waveguides, where the photonic-crystal lattice on each side of a guiding region is shifted in the longitudinal direction by half a period, thereby possessing glide-plane symmetry. The type B waveguides are utilized to reduce out-of-plane radiation losses of linear polarized light propagating through the waveguide.
Peter Lodahl et al.: “Interfacing single photons and single quantum dots with photonic nanostructures”, 4 Dec. 2013, ArXiv, is a disclosure by the present inventor group that provides an overview of quantum optics with excitons in single quantum dots embedded in photonic nanostructures.
Overall, there is still a need for waveguides that allow for on-chip efficiently read-out of quantum emitters having circularly polarized transition dipole moments.